Many of the clinical research projects of the laser and modern optics group involve the interaction of light with tissue (e.g., low-coherence interferometric microscopy, diffusive-wave and time-gated optical imaging, laser microsurgery, laser Doppler blood-flow measurements, photodynamic therapy of cancer, noninvasive platelet assessment, and optical tissue oximetry). In order to quantitate these techniques more fully and to develop new quantitative, noninvasive techniques for tissue spectroscopy,, we have undertaken theoretical modeling of light propagation in biological tissues and turbid media. Analytical equations have been devised to characterize various parameters of photons illuminating a tissue surface (including probability of surface re-emission at a given distance, mean path before re-emission, mean depth of penetration, and probability of absorption with depth). These expressions have been used to interpret empirical measurements on living tissues and to quantify a variety of clinical measurements (e.g., laser Doppler blood-flow and volume measurements, dosimetry in PDT of cancer, and remote sensing of tissue structure and chemical abnormalities). Recently our theoretical predictions of pathlength distributions of re-emitted photons have been applied to temporal dispersion of picosecond laser pulses in breast, brain, and muscle. Such analysis may allow noninvasive, absolute quantitation of hemoglobin and myoglobin oxygen saturation in vivo, as well as the concentration of a variety of important biomolecules. We have definitely demonstrated the impracticality of time-resolved optical mammography as a primary screening tool.